util/concurrent/FutureTask.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;
import java.util.concurrent.locks.LockSupport;

/**
 * A cancellable asynchronous computation.  This class provides a base
 * implementation of {@link Future}, with methods to start and cancel
 * a computation, query to see if the computation is complete, and
 * retrieve the result of the computation.  The result can only be
 * retrieved when the computation has completed; the {@code get}
 * methods will block if the computation has not yet completed.  Once
 * the computation has completed, the computation cannot be restarted
 * or cancelled (unless the computation is invoked using
 * {@link #runAndReset}).
 *
 * <p>A {@code FutureTask} can be used to wrap a {@link Callable} or
 * {@link Runnable} object.  Because {@code FutureTask} implements
 * {@code Runnable}, a {@code FutureTask} can be submitted to an
 * {@link Executor} for execution.
 *
 * <p>In addition to serving as a standalone class, this class provides
 * {@code protected} functionality that may be useful when creating
 * customized task classes.
 *
 * @since 1.5
 * @author Doug Lea
 * @param <V> The result type returned by this FutureTask's {@code get} methods
 */
public class FutureTask<V> implements RunnableFuture<V> {
    /*
     * Revision notes: This differs from previous versions of this
     * class that relied on AbstractQueuedSynchronizer, mainly to
     * avoid surprising users about retaining interrupt status during
     * cancellation races. Sync control in the current design relies
     * on a "state" field updated via CAS to track completion, along
     * with a simple Treiber stack to hold waiting threads.
     *
     * Style note: As usual, we bypass overhead of using
     * AtomicXFieldUpdaters and instead directly use Unsafe intrinsics.
     */

    /**
     * The run state of this task, initially UNDECIDED.  The run state
     * transitions to NORMAL, EXCEPTIONAL, or CANCELLED (only) in
     * method setCompletion.  During setCompletion, state may take on
     * transient values of COMPLETING (while outcome is being set) or
     * INTERRUPTING (while interrupting the runner).  State values are
     * ordered and set to powers of two to simplify checks.
     */
    private volatile int state;
    private static final int UNDECIDED    = 0x00;
    private static final int COMPLETING   = 0x01;
    private static final int INTERRUPTING = 0x02;
    private static final int NORMAL       = 0x04;
    private static final int EXCEPTIONAL  = 0x08;
    private static final int CANCELLED    = 0x10;

    /** The underlying callable */
    private final Callable<V> callable;
    /** The result to return or exception to throw from get() */
    private Object outcome; // non-volatile, protected by state reads/writes
    /** The thread running the callable; CASed during run() */
    private volatile Thread runner;
    /** Treiber stack of waiting threads */
    private volatile WaitNode waiters;

    /**
     * Sets completion status, unless already completed.  If
     * necessary, we first set state to COMPLETING or INTERRUPTING to
     * establish precedence.  This intentionally stalls (just via
     * yields) in (uncommon) cases of concurrent calls during
     * cancellation until state is set, to avoid surprising users
     * during cancellation races.
     *
     * @param x the outcome
     * @param mode the completion state value
     * @return true if this call caused transition from UNDECIDED to completed
     */
    private boolean setCompletion(Object x, int mode) {
        int next = ((mode == INTERRUPTING) ? // set up transient states
                    (runner != null) ? INTERRUPTING : CANCELLED :
                    (x != null) ? COMPLETING : mode);
        if (!UNSAFE.compareAndSwapInt(this, stateOffset,
                                      UNDECIDED, next))
            return false;
        if (next == INTERRUPTING) {
            Thread t = runner; // recheck
            if (t != null)
                t.interrupt();
            state = CANCELLED;
        }
        else if (next == COMPLETING) {
            outcome = x;
            state = mode;
        }
        if (waiters != null)
            releaseAll();
        done();
        return true;
    }

    /**
     * Returns result or throws exception for completed task.
     *
     * @param s completed state value
     */
    private V report(int s) throws ExecutionException {
        Object x = outcome;
        if (s == NORMAL)
            return (V)x;
        if ((s & (CANCELLED | INTERRUPTING)) != 0)
            throw new CancellationException();
        throw new ExecutionException((Throwable)x);
    }

    /**
     * Creates a {@code FutureTask} that will, upon running, execute the
     * given {@code Callable}.
     *
     * @param  callable the callable task
     * @throws NullPointerException if callable is null
     */
    public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
    }

    /**
     * Creates a {@code FutureTask} that will, upon running, execute the
     * given {@code Runnable}, and arrange that {@code get} will return the
     * given result on successful completion.
     *
     * @param runnable the runnable task
     * @param result the result to return on successful completion. If
     * you don't need a particular result, consider using
     * constructions of the form:
     * {@code Future<?> f = new FutureTask<Void>(runnable, null)}
     * @throws NullPointerException if runnable is null
     */
    public FutureTask(Runnable runnable, V result) {
        this.callable = Executors.callable(runnable, result);
    }

    public boolean isCancelled() {
        return (state & (CANCELLED | INTERRUPTING)) != 0;
    }

    public boolean isDone() {
        return state != UNDECIDED;
    }

    public boolean cancel(boolean mayInterruptIfRunning) {
        return state == UNDECIDED &&
            setCompletion(null, mayInterruptIfRunning ?
                          INTERRUPTING : CANCELLED);
    }

    /**
     * @throws CancellationException {@inheritDoc}
     */
    public V get() throws InterruptedException, ExecutionException {
        int s = state;
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);
        return report(s);
    }

    /**
     * @throws CancellationException {@inheritDoc}
     */
    public V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException {
        int s = state;
        if (s <= COMPLETING &&
            (s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
            throw new TimeoutException();
        return report(s);
    }

    /**
     * Protected method invoked when this task transitions to state
     * {@code isDone} (whether normally or via cancellation). The
     * default implementation does nothing.  Subclasses may override
     * this method to invoke completion callbacks or perform
     * bookkeeping. Note that you can query status inside the
     * implementation of this method to determine whether this task
     * has been cancelled.
     */
    protected void done() { }

    /**
     * Sets the result of this future to the given value unless
     * this future has already been set or has been cancelled.
     *
     * <p>This method is invoked internally by the {@link #run} method
     * upon successful completion of the computation.
     *
     * @param v the value
     */
    protected void set(V v) {
        setCompletion(v, NORMAL);
    }

    /**
     * Causes this future to report an {@link ExecutionException}
     * with the given throwable as its cause, unless this future has
     * already been set or has been cancelled.
     *
     * <p>This method is invoked internally by the {@link #run} method
     * upon failure of the computation.
     *
     * @param t the cause of failure
     */
    protected void setException(Throwable t) {
        setCompletion(t, EXCEPTIONAL);
    }

    public void run() {
        if (state != UNDECIDED ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return;

        try {
            V result;
            try {
                result = callable.call();
            } catch (Throwable ex) {
                setException(ex);
                return;
            }
            set(result);
        } finally {
            runner = null;
            while (state == INTERRUPTING)
                Thread.yield(); // wait out pending cancellation interrupt
        }
    }

    /**
     * Executes the computation without setting its result, and then
     * resets this future to initial state, failing to do so if the
     * computation encounters an exception or is cancelled.  This is
     * designed for use with tasks that intrinsically execute more
     * than once.
     *
     * @return true if successfully run and reset
     */
    protected boolean runAndReset() {
        if (state != UNDECIDED ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return false;

        try {
            try {
                callable.call(); // don't set result
                return (state == UNDECIDED);
            } catch (Throwable ex) {
                setException(ex);
                return false;
            }
        } finally {
            runner = null;
            while (state == INTERRUPTING)
                Thread.yield(); // wait out pending cancellation interrupt
        }
    }

    /**
     * Simple linked list nodes to record waiting threads in a Treiber
     * stack.  See other classes such as Phaser and SynchronousQueue
     * for more detailed explanation.
     */
    static final class WaitNode {
        volatile Thread thread;
        WaitNode next;
    }

    /**
     * Removes and signals all waiting threads.
     */
    private void releaseAll() {
        WaitNode q;
        while ((q = waiters) != null) {
            if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
                for (;;) {
                    Thread t = q.thread;
                    if (t != null) {
                        q.thread = null;
                        LockSupport.unpark(t);
                    }
                    WaitNode next = q.next;
                    if (next == null)
                        return;
                    q.next = null; // unlink to help gc
                    q = next;
                }
            }
        }
    }

    /**
     * Awaits completion or aborts on interrupt or timeout.
     *
     * @param timed true if use timed waits
     * @param nanos time to wait, if timed
     * @return state upon completion
     */
    private int awaitDone(boolean timed, long nanos)
        throws InterruptedException {
        long last = timed ? System.nanoTime() : 0L;
        WaitNode q = null;
        boolean queued = false;
        for (;;) {
            if (Thread.interrupted()) {
                removeWaiter(q);
                throw new InterruptedException();
            }

            int s = state;
            if (s > COMPLETING) {
                if (q != null)
                    q.thread = null;
                return s;
            }
            else if (q == null)
                q = new WaitNode();
            else if (!queued)
                queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                     q.next = waiters, q);
            else if (q.thread == null)
                q.thread = Thread.currentThread();
            else if (timed) {
                long now = System.nanoTime();
                if ((nanos -= (now - last)) <= 0L) {
                    removeWaiter(q);
                    return state;
                }
                last = now;
                LockSupport.parkNanos(this, nanos);
            }
            else
                LockSupport.park(this);
        }
    }

    /**
     * Tries to unlink a timed-out or interrupted wait node to avoid
     * accumulating garbage.  Internal nodes are simply unspliced
     * without CAS since it is harmless if they are traversed anyway
     * by releasers or concurrent calls to removeWaiter.
     */
    private void removeWaiter(WaitNode node) {
        if (node != null) {
            node.thread = null;
            WaitNode pred = null;
            WaitNode q = waiters;
            while (q != null) {
                WaitNode next = node.next;
                if (q != node) {
                    pred = q;
                    q = next;
                }
                else if (pred != null) {
                    pred.next = next;
                    break;
                }
                else if (UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                     q, next))
                    break;
                else { // restart on CAS failure
                    pred = null;
                    q = waiters;
                }
            }
        }
    }

    // Unsafe mechanics
    private static final sun.misc.Unsafe UNSAFE;
    private static final long stateOffset;
    private static final long runnerOffset;
    private static final long waitersOffset;
    static {
        try {
            UNSAFE = sun.misc.Unsafe.getUnsafe();
            Class<?> k = FutureTask.class;
            stateOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("state"));
            runnerOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("runner"));
            waitersOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("waiters"));
        } catch (Exception e) {
            throw new Error(e);
        }
    }

}
Revision:	1.68
Committed:	Fri Jun 17 22:07:24 2011 UTC (12 years, 11 months ago) by jsr166
Branch:	MAIN
Changes since 1.67:	+38 -44 lines
Log Message:	only reset runner to null after state is decided
#	User	Rev	Content
1	tim	1.1	/*
2	dl	1.2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	dl	1.23	* Expert Group and released to the public domain, as explained at
4	jsr166	1.59	* http://creativecommons.org/publicdomain/zero/1.0/
5	tim	1.1	*/
6
7			package java.util.concurrent;
8	dl	1.62	import java.util.concurrent.locks.LockSupport;
9	dl	1.13
10	tim	1.1	/**
11	dl	1.8	* A cancellable asynchronous computation. This class provides a base
12			* implementation of {@link Future}, with methods to start and cancel
13			* a computation, query to see if the computation is complete, and
14	dl	1.4	* retrieve the result of the computation. The result can only be
15	jsr166	1.64	* retrieved when the computation has completed; the {@code get}
16			* methods will block if the computation has not yet completed. Once
17	dl	1.8	* the computation has completed, the computation cannot be restarted
18	jsr166	1.64	* or cancelled (unless the computation is invoked using
19			* {@link #runAndReset}).
20	tim	1.1	*
21	jsr166	1.64	* <p>A {@code FutureTask} can be used to wrap a {@link Callable} or
22			* {@link Runnable} object. Because {@code FutureTask} implements
23			* {@code Runnable}, a {@code FutureTask} can be submitted to an
24			* {@link Executor} for execution.
25	tim	1.1	*
26	dl	1.14	* <p>In addition to serving as a standalone class, this class provides
27	jsr166	1.64	* {@code protected} functionality that may be useful when creating
28	dl	1.14	* customized task classes.
29			*
30	tim	1.1	* @since 1.5
31	dl	1.4	* @author Doug Lea
32	jsr166	1.64	* @param <V> The result type returned by this FutureTask's {@code get} methods
33	tim	1.1	*/
34	peierls	1.39	public class FutureTask<V> implements RunnableFuture<V> {
35	dl	1.62	/*
36			* Revision notes: This differs from previous versions of this
37			* class that relied on AbstractQueuedSynchronizer, mainly to
38			* avoid surprising users about retaining interrupt status during
39			* cancellation races. Sync control in the current design relies
40			* on a "state" field updated via CAS to track completion, along
41			* with a simple Treiber stack to hold waiting threads.
42			*
43			* Style note: As usual, we bypass overhead of using
44			* AtomicXFieldUpdaters and instead directly use Unsafe intrinsics.
45			*/
46
47			/**
48	jsr166	1.67	* The run state of this task, initially UNDECIDED. The run state
49	dl	1.62	* transitions to NORMAL, EXCEPTIONAL, or CANCELLED (only) in
50	jsr166	1.67	* method setCompletion. During setCompletion, state may take on
51	dl	1.62	* transient values of COMPLETING (while outcome is being set) or
52	jsr166	1.67	* INTERRUPTING (while interrupting the runner). State values are
53			* ordered and set to powers of two to simplify checks.
54	dl	1.62	*/
55			private volatile int state;
56	jsr166	1.67	private static final int UNDECIDED = 0x00;
57	dl	1.62	private static final int COMPLETING = 0x01;
58			private static final int INTERRUPTING = 0x02;
59			private static final int NORMAL = 0x04;
60			private static final int EXCEPTIONAL = 0x08;
61			private static final int CANCELLED = 0x10;
62
63	jsr166	1.64	/** The underlying callable */
64			private final Callable<V> callable;
65	dl	1.62	/** The result to return or exception to throw from get() */
66			private Object outcome; // non-volatile, protected by state reads/writes
67			/** The thread running the callable; CASed during run() */
68			private volatile Thread runner;
69			/** Treiber stack of waiting threads */
70			private volatile WaitNode waiters;
71
72			/**
73			* Sets completion status, unless already completed. If
74			* necessary, we first set state to COMPLETING or INTERRUPTING to
75	jsr166	1.64	* establish precedence. This intentionally stalls (just via
76	dl	1.62	* yields) in (uncommon) cases of concurrent calls during
77			* cancellation until state is set, to avoid surprising users
78			* during cancellation races.
79			*
80			* @param x the outcome
81			* @param mode the completion state value
82	jsr166	1.67	* @return true if this call caused transition from UNDECIDED to completed
83	dl	1.62	*/
84			private boolean setCompletion(Object x, int mode) {
85			int next = ((mode == INTERRUPTING) ? // set up transient states
86	jsr166	1.68	(runner != null) ? INTERRUPTING : CANCELLED :
87	dl	1.62	(x != null) ? COMPLETING : mode);
88	jsr166	1.68	if (!UNSAFE.compareAndSwapInt(this, stateOffset,
89			UNDECIDED, next))
90			return false;
91			if (next == INTERRUPTING) {
92			Thread t = runner; // recheck
93			if (t != null)
94			t.interrupt();
95			state = CANCELLED;
96			}
97			else if (next == COMPLETING) {
98			outcome = x;
99			state = mode;
100	dl	1.62	}
101	jsr166	1.68	if (waiters != null)
102			releaseAll();
103			done();
104			return true;
105	dl	1.62	}
106
107			/**
108	jsr166	1.64	* Returns result or throws exception for completed task.
109			*
110	dl	1.62	* @param s completed state value
111			*/
112			private V report(int s) throws ExecutionException {
113			Object x = outcome;
114			if (s == NORMAL)
115			return (V)x;
116			if ((s & (CANCELLED \| INTERRUPTING)) != 0)
117			throw new CancellationException();
118			throw new ExecutionException((Throwable)x);
119			}
120	dl	1.11
121	tim	1.1	/**
122	jsr166	1.64	* Creates a {@code FutureTask} that will, upon running, execute the
123			* given {@code Callable}.
124	tim	1.1	*
125			* @param callable the callable task
126	dl	1.9	* @throws NullPointerException if callable is null
127	tim	1.1	*/
128			public FutureTask(Callable<V> callable) {
129	dl	1.9	if (callable == null)
130			throw new NullPointerException();
131	dl	1.62	this.callable = callable;
132	tim	1.1	}
133
134			/**
135	jsr166	1.64	* Creates a {@code FutureTask} that will, upon running, execute the
136			* given {@code Runnable}, and arrange that {@code get} will return the
137	tim	1.1	* given result on successful completion.
138			*
139	jsr166	1.54	* @param runnable the runnable task
140	tim	1.1	* @param result the result to return on successful completion. If
141	dl	1.9	* you don't need a particular result, consider using
142	dl	1.16	* constructions of the form:
143	jsr166	1.58	* {@code Future<?> f = new FutureTask<Void>(runnable, null)}
144	dl	1.9	* @throws NullPointerException if runnable is null
145	tim	1.1	*/
146	dl	1.15	public FutureTask(Runnable runnable, V result) {
147	dl	1.62	this.callable = Executors.callable(runnable, result);
148	dl	1.20	}
149
150			public boolean isCancelled() {
151	dl	1.62	return (state & (CANCELLED \| INTERRUPTING)) != 0;
152	dl	1.20	}
153	jsr166	1.35
154	dl	1.20	public boolean isDone() {
155	jsr166	1.67	return state != UNDECIDED;
156	dl	1.13	}
157
158			public boolean cancel(boolean mayInterruptIfRunning) {
159	jsr166	1.67	return state == UNDECIDED &&
160	dl	1.62	setCompletion(null, mayInterruptIfRunning ?
161			INTERRUPTING : CANCELLED);
162	dl	1.13	}
163	jsr166	1.35
164	jsr166	1.43	/**
165			* @throws CancellationException {@inheritDoc}
166			*/
167	dl	1.2	public V get() throws InterruptedException, ExecutionException {
168	jsr166	1.64	int s = state;
169			if (s <= COMPLETING)
170			s = awaitDone(false, 0L);
171			return report(s);
172	tim	1.1	}
173
174	jsr166	1.43	/**
175			* @throws CancellationException {@inheritDoc}
176			*/
177	dl	1.2	public V get(long timeout, TimeUnit unit)
178	tim	1.1	throws InterruptedException, ExecutionException, TimeoutException {
179	jsr166	1.64	int s = state;
180			if (s <= COMPLETING &&
181			(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
182	dl	1.62	throw new TimeoutException();
183			return report(s);
184	tim	1.1	}
185
186			/**
187	dl	1.20	* Protected method invoked when this task transitions to state
188	jsr166	1.64	* {@code isDone} (whether normally or via cancellation). The
189	dl	1.20	* default implementation does nothing. Subclasses may override
190			* this method to invoke completion callbacks or perform
191			* bookkeeping. Note that you can query status inside the
192			* implementation of this method to determine whether this task
193			* has been cancelled.
194			*/
195			protected void done() { }
196
197			/**
198	jsr166	1.64	* Sets the result of this future to the given value unless
199	dl	1.29	* this future has already been set or has been cancelled.
200	jsr166	1.64	*
201			* <p>This method is invoked internally by the {@link #run} method
202	dl	1.40	* upon successful completion of the computation.
203	jsr166	1.64	*
204	tim	1.1	* @param v the value
205	jsr166	1.35	*/
206	dl	1.2	protected void set(V v) {
207	dl	1.62	setCompletion(v, NORMAL);
208	tim	1.1	}
209
210			/**
211	jsr166	1.64	* Causes this future to report an {@link ExecutionException}
212			* with the given throwable as its cause, unless this future has
213	dl	1.24	* already been set or has been cancelled.
214	jsr166	1.64	*
215			* <p>This method is invoked internally by the {@link #run} method
216	dl	1.40	* upon failure of the computation.
217	jsr166	1.64	*
218	jsr166	1.41	* @param t the cause of failure
219	jsr166	1.35	*/
220	dl	1.2	protected void setException(Throwable t) {
221	dl	1.62	setCompletion(t, EXCEPTIONAL);
222	tim	1.1	}
223	jsr166	1.35
224	dl	1.24	public void run() {
225	jsr166	1.68	if (state != UNDECIDED \|\|
226			!UNSAFE.compareAndSwapObject(this, runnerOffset,
227			null, Thread.currentThread()))
228			return;
229
230			try {
231	dl	1.62	V result;
232			try {
233			result = callable.call();
234			} catch (Throwable ex) {
235			setException(ex);
236			return;
237			}
238			set(result);
239	jsr166	1.68	} finally {
240			runner = null;
241			while (state == INTERRUPTING)
242			Thread.yield(); // wait out pending cancellation interrupt
243	dl	1.62	}
244	dl	1.24	}
245
246			/**
247	dl	1.30	* Executes the computation without setting its result, and then
248	jsr166	1.64	* resets this future to initial state, failing to do so if the
249	dl	1.24	* computation encounters an exception or is cancelled. This is
250			* designed for use with tasks that intrinsically execute more
251			* than once.
252	jsr166	1.64	*
253	dl	1.24	* @return true if successfully run and reset
254			*/
255			protected boolean runAndReset() {
256	jsr166	1.67	if (state != UNDECIDED \|\|
257	jsr166	1.66	!UNSAFE.compareAndSwapObject(this, runnerOffset,
258			null, Thread.currentThread()))
259	dl	1.62	return false;
260	jsr166	1.68
261	dl	1.62	try {
262	jsr166	1.68	try {
263			callable.call(); // don't set result
264			return (state == UNDECIDED);
265			} catch (Throwable ex) {
266			setException(ex);
267	dl	1.62	return false;
268	jsr166	1.68	}
269			} finally {
270			runner = null;
271			while (state == INTERRUPTING)
272			Thread.yield(); // wait out pending cancellation interrupt
273	dl	1.62	}
274	dl	1.14	}
275	dl	1.3
276	dl	1.14	/**
277	dl	1.62	* Simple linked list nodes to record waiting threads in a Treiber
278	jsr166	1.64	* stack. See other classes such as Phaser and SynchronousQueue
279	dl	1.62	* for more detailed explanation.
280	dl	1.20	*/
281	dl	1.62	static final class WaitNode {
282			volatile Thread thread;
283			WaitNode next;
284			}
285	dl	1.42
286	dl	1.62	/**
287	jsr166	1.64	* Removes and signals all waiting threads.
288	dl	1.62	*/
289			private void releaseAll() {
290			WaitNode q;
291			while ((q = waiters) != null) {
292			if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
293			for (;;) {
294			Thread t = q.thread;
295			if (t != null) {
296			q.thread = null;
297			LockSupport.unpark(t);
298			}
299			WaitNode next = q.next;
300			if (next == null)
301			return;
302			q.next = null; // unlink to help gc
303			q = next;
304			}
305			}
306	dl	1.24	}
307	dl	1.62	}
308	dl	1.24
309	dl	1.62	/**
310	jsr166	1.64	* Awaits completion or aborts on interrupt or timeout.
311			*
312	dl	1.62	* @param timed true if use timed waits
313	jsr166	1.64	* @param nanos time to wait, if timed
314	dl	1.62	* @return state upon completion
315			*/
316			private int awaitDone(boolean timed, long nanos)
317			throws InterruptedException {
318	jsr166	1.63	long last = timed ? System.nanoTime() : 0L;
319	dl	1.62	WaitNode q = null;
320			boolean queued = false;
321	jsr166	1.64	for (;;) {
322	dl	1.62	if (Thread.interrupted()) {
323			removeWaiter(q);
324			throw new InterruptedException();
325			}
326	jsr166	1.64
327			int s = state;
328			if (s > COMPLETING) {
329	dl	1.62	if (q != null)
330			q.thread = null;
331			return s;
332			}
333			else if (q == null)
334			q = new WaitNode();
335			else if (!queued)
336			queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
337			q.next = waiters, q);
338			else if (q.thread == null)
339			q.thread = Thread.currentThread();
340			else if (timed) {
341			long now = System.nanoTime();
342			if ((nanos -= (now - last)) <= 0L) {
343			removeWaiter(q);
344			return state;
345	dl	1.50	}
346	dl	1.62	last = now;
347			LockSupport.parkNanos(this, nanos);
348	dl	1.50	}
349	dl	1.62	else
350			LockSupport.park(this);
351	dl	1.24	}
352	dl	1.62	}
353	dl	1.24
354	dl	1.62	/**
355	jsr166	1.64	* Tries to unlink a timed-out or interrupted wait node to avoid
356			* accumulating garbage. Internal nodes are simply unspliced
357	dl	1.62	* without CAS since it is harmless if they are traversed anyway
358			* by releasers or concurrent calls to removeWaiter.
359			*/
360			private void removeWaiter(WaitNode node) {
361			if (node != null) {
362			node.thread = null;
363			WaitNode pred = null;
364			WaitNode q = waiters;
365			while (q != null) {
366			WaitNode next = node.next;
367			if (q != node) {
368			pred = q;
369			q = next;
370	dl	1.50	}
371	dl	1.62	else if (pred != null) {
372			pred.next = next;
373			break;
374	dl	1.50	}
375	dl	1.62	else if (UNSAFE.compareAndSwapObject(this, waitersOffset,
376			q, next))
377	jsr166	1.56	break;
378	dl	1.62	else { // restart on CAS failure
379			pred = null;
380			q = waiters;
381	jsr166	1.55	}
382	jsr166	1.56	}
383	dl	1.14	}
384	dl	1.62	}
385	dl	1.14
386	dl	1.62	// Unsafe mechanics
387			private static final sun.misc.Unsafe UNSAFE;
388			private static final long stateOffset;
389			private static final long runnerOffset;
390			private static final long waitersOffset;
391			static {
392			try {
393			UNSAFE = sun.misc.Unsafe.getUnsafe();
394			Class<?> k = FutureTask.class;
395			stateOffset = UNSAFE.objectFieldOffset
396			(k.getDeclaredField("state"));
397			runnerOffset = UNSAFE.objectFieldOffset
398			(k.getDeclaredField("runner"));
399			waitersOffset = UNSAFE.objectFieldOffset
400			(k.getDeclaredField("waiters"));
401			} catch (Exception e) {
402			throw new Error(e);
403	dl	1.14	}
404	dl	1.15	}
405	dl	1.62
406	dl	1.15	}